A number of configurations of fixed-wing VTOL aircraft have been proposed in the art. See, for example, U.S. Pat. Nos. 3,618,875 and 3,388,878. The aircraft in those two patents utilize several lifting fans to elevate the aircraft off the ground and separate engines for forward thrust to produce lift-generating forward air speed. The lifting fans according to both references are powered by gas bled from the thrust-producing engines using fairly complex, and therefore fairly heavy, take-off ducting. As far as I know, no aircraft according to either reference has ever been built and flown successfully.
Rather, as far as I know, only two fixed-wing VTOL aircraft have been built and flown successfully in any meaningful numbers, namely, the AV8-B/Harrier “Jumpjet” and the Lockheed-Martin X-35 “Joint Strike Fighter” (currently in development). Although those two aircraft have proven themselves successfully, they both suffer from a number of design drawbacks or sensitivities. Using the Harrier for illustration purposes, I explain these deficiencies below.
As illustrated in FIG. 1A, at low forward velocity, airflow over the root of the wing is generally laminar. Flow over the mid-span portions of the wing, however, tends to be less stable, and flow over the tip portions of the wing becomes highly unstable to turbulent. As forward velocity increases, the region of laminar flow tends to spread or progress from the root of the wing toward the tip of the wing. The flow over the tip region of the wing, however, remains somewhat unstable. It is not until the aircraft reaches forward airspeeds on the order of 120 knots that flow along the entire span of the wing becomes laminar.
As one having skill in the art will appreciate, the greater the moment arm of the control forces that can be generated, the greater the degree of aircraft stability that can be maintained with the control forces. Therefore, because the flow over the wing of a conventional fixed-wing VTOL aircraft such as the Harrier is laminar generally only near the root of the wing in low-speed forward flight, control forces are generated only with relatively short moment arms relative to the aircraft datum center line in low-speed forward flight. Therefore, as the aircraft transitions from pure vertical take-off to forward motion, roll control is relatively “soft” or “mushy.” Only after the conventional fixed-wing VTOL aircraft has attained a forward speed on the order of 70 knots will full roll controllability of the wing be realized. (To compensate for this, Harriers use a series of jet thrusters at their wingtips, which utilize bleed air from the engine; this adds complexity and weight and reduces thrusting power available for lift-off.)
In addition to the delay in achieving strong positive roll control, the configuration of a conventional fixed-wing VTOL aircraft also makes pitch control somewhat difficult. As the aircraft gains forward velocity and laminar flow progresses toward the wingtip, which is located rearward of the root, the mean aerodynamic center of pressure or center of lift progresses rearward. Because the mean aerodynamic center is located behind the center of mass of the aircraft, as the mean aerodynamic center moves rearward, it moves further away from the center of mass of the aircraft and its moment arm increases. Therefore, as the aircraft increases its forward velocity and the mean aerodynamic center moves further rearward, the pitch control surfaces have to compensate for this increased moment arm. This also increases static longitudinal instability of the aircraft. Therefore, it becomes more difficult to control the vehicle in transition from VTOL through horizontal flight through pitch control inputs.
Another phenomenon which occurs in conventional fixed-wing VTOL aircraft such as the Harrier is lift entrainment or “suck down,” which is a phenomenon that occurs within about four feet to twelve feet of the ground and that is related to ground effect. In particular, in a conventional fixed-wing VTOL aircraft such as the Harrier, vertical lift-off is achieved by rotating two sets of nozzles to direct gas flow from the engine downwardly. The conventional configuration has those nozzles positioned generally right under the wings of the aircraft, as illustrated in FIGS. 1B and 1C. As further illustrated in those Figures, when the jet exhaust strikes the ground, it is diverted laterally and flows horizontally along the ground under the wings. Because the flow is hot gas, as soon as the gas is able to rise, i.e., as soon as it exits out from under the wings, it does so. Cooler, heavier air above, however, forces the flow back down on top of the wing, creating the circular flow pattern illustrated in FIG. 1B. Thus, there is increased pressure force on top of the wing, pressing the wing downward. Additionally, the horizontal, lateral flow of gas along the underside of the wing illustrated in FIG. 1C reduces pressure due to Bernoulli's principle, and that reduced pressure under the wing creates a suction force which retards vertical lift-off of the aircraft.
Thus, although the AV8-B/Harrier has been a generally successful fixed-wing VTOL program, and although the X-35 “Joint Strike Fighter” also should be a successful program, conventional fixed-wing VTOL aircraft are not without their design difficulties. As a result, they can be somewhat difficult or tricky aircraft to fly.